1. Field of the Invention
Biaxially oriented polyester films are used in packaging and in industry primarily where there is a need for their advantageous properties, i.e. good optical properties, high mechanical strengths, good barrier effect in particular against gases, good dimensional stability when heated and excellent layflat.
In food packaging applications, packaging technology requires a high barrier effect against gases, steam and flavors (this having the same significance as low transmission or low permeability). A well-known process for producing packaging of this type consists in high-vacuum aluminum metalizing of the plastic films used for this purpose. Other well-known processes include coating the films with oxidic materials (e.g. SiO.sub.x or Al.sub.x O.sub.y) or water glass. Essentially, the coatings used are transparent.
The barrier effect against the substances mentioned above depends essentially on the type of the polymers in the film and the quality of the barrier layers applied. Thus, a very high barrier effect against gases, such as oxygen and flavors, is achieved in metalized, biaxially oriented polyester films. A barrier effect against steam is achieved in metalized, biaxially oriented polypropylene films.
The good barrier properties of metalized or oxidically coated films mean that they are used in particular for packaging foodstuffs and other consumable items, for which long storage or transport times create the risk that the packaged foodstuffs lose flavor or become spoiled or rancid if there is an inadequate barrier. Examples of such foodstuffs and consumable items include coffee, snacks containing fats (nuts, potato chips, etc.), and drinks containing carbon dioxide (in pouches).
If polyester films metalized with an aluminum layer or having applied oxidic layers are used as packaging material, they are generally a constituent of a multilayer composite film (laminate). Bags produced therefrom can be filled, for example, on a vertical tubular bag forming, filling and sealing machine. The bags are heat-sealed on their inward side (i.e. on the side facing the contents), the heat-sealable layer consisting, for example, of polyethylene. The composite film here typically has the following structure: polyester layer/aluminum or oxide layer/adhesive layer/heat-sealable layer. The thickness of the metal or oxide layer is only from 10 to 80 nm. Even this very thin functional layer is sufficiently effective to achieve adequate protection from light and very good barrier properties.
The oxygen barrier or the oxygen transmission is generally measured not on the laminate or the packaging itself, but on the metalized polyester film. To ensure good quality of the foodstuffs or other consumable items even after relatively long storage times, the oxygen transmission (identical to permeability) of the metalized film may not be greater than 2 cm.sup.3 /(m.sup.2 bar d), but in particular not greater than 1.5 cm.sup.3 /(m.sup.2 bar d). In the future, the demands of the packaging industry will head toward still higher barriers, with attempts to achieve permeability values of less than 1.0 cm.sup.3 /(m.sup.2 bar d) for metalized films.
2. Description of Related Arts
In the prior art, there is neither sufficient knowledge of the detailed basis for the barrier effect of metalized or oxidically coated biaxially oriented polyester films nor of how this may be decisively improved. Variables which are clearly important are the substrate surface, and the substrate polymer and its morphology.
Weiss et al., in "Thin Solids Films" 204 (1991), p. 203-216, studied the influence of the roughness of a substrate layer on permeability. In this study, polyester films were coated with lacquer which contained various concentrations of titanium dioxide particles. In the experiments described, the concentration of titanium dioxide particles in the coating varied from 2 to 20% by weight. Using this method, the roughness R.sub.a of the coated substrate surface could be varied from 43 nm (uncoated and coated film, without titanium dioxide) to 124 nm. In his experiments, increasing roughness (increasing proportion of TiO.sub.2) of the coated surface resulted in markedly higher oxygen transmissions after metalizing with aluminum. However, the largest step increase in oxygen transmission was seen when the coated film (0% by weight) was compared with the uncoated film, although the surface roughness of the substrate surface was the same in both cases. Merely coating the film gave a deterioration in the barrier from about 0.43 cm.sup.3 /(m.sup.2 d bar) (plain film) to about 19 cm.sup.3 /(m.sup.2 d bar) (coated film). A further uncertainty concerning the transferability of this work to commercial products is created by the fact that the aluminum layer was applied using a laboratory evaporator. When compared with an industrial metalizer, this method achieves essentially low permeability values, and the influence of the substrate surface on the barrier properties cannot be clearly seen.
Other detailed results of studies on the influence of the substrate surface of polyester films on their barrier properties can be found in the dissertation by H. Utz (Technische Universitat Munchen 1995: "Barriereeigenschaften Aluminiumbedampfter Kunststoffolien" [Barrier Properties of Aluminum-Metalized Plastic Films]).
EP-A-0 490 665 A1 describes a single-layer biaxially oriented polyester film for magnetic recording tape; the film contains
a) from 0.05 to 1.0% by weight of .omega.-alumina having an average particle diameter in the range from 0.02 to 0.3 .mu.m, and PA1 b) from 0.01 to 1.5% by weight of inert particles of a type other than .omega.-alumina and having an average particle diameter in the range from 0.1 to 1.5 .mu.m, these particles being larger than the .omega.-alumina particles.
The surface of this film is formed by a large number of elevations/protrusions which are described by the relationship EQU -11.4x+4&lt;log y&lt;-10.0x+5 where y&gt;30, x&gt;0.05 .mu.m.
In this equation, x (.mu.m) is a height above a standard level and y is the number of elevations (number/mm.sup.2) if the elevations are sectioned at a height of x. The distribution of the elevations is measured with a standard apparatus for measuring roughness. This text gives no information concerning improvement of the barrier properties, the gloss or the haze.
It is also known that the oxygen barrier can be improved by selecting particular polymers for the film serving as substrate (Schrikker, G.: Metallisierte Kunststoffolien fur Hoherwertige Verpackungen [Metalized Plastic Films for High-Quality Packaging] in ICI 5th International Metallising Symposium 1986, Cannes). Polyesters, for example, are particularly suitable, specifically those made from ethyleneglycol and terephthalic acid or from ethyleneglycol, terephthalic acid and naphthalene-2,6-dicarboxylic acid. Besides these, polyamides, ethylene-vinyl alcohol copolymers (EVOH) and polyvinylidene chloride may also be used with advantage. Thus, for example, U.S. Pat. No. 5,506,014 describes a copolyester made from (a) from 45 to 85 mol% of terephthalic acid, (b) from 10 to 40 mol% of naphthalenedicarboxylic acid and (c) from 5 to 15 mol% of a dicarboxylic acid having from 2 to 8 carbon atoms and (d) ethyleneglycol; (the molar percentages are based on the total proportion of dicarboxylic acids). This polyester is claimed to have better barrier properties against gases. It is used, inter alia, for producing bottles or containers and films of various thicknesses. A disadvantage of the raw materials mentioned is that they are significantly more expensive than polyethylene terephthalate (PET) or are unsuitable and/or not officially permitted for use in food packaging applications.